The researcher would be interested in examining the relationship
between two variables. So that he can know what type of
relationship exists in between them? Whether any change in one
variable may influence positively or negatively to the movement of
another variable.

For example, economics theory says that price and quantity
demand for a commodity is inversely related. Any change in price of
a commodity influence the quantity demand for that commodity. If
price of commodity increase (decrease) the quantity demand for that
commodity will decrease (increase). So examining the relationship
will help us to take appropriate decision. Moreover, it also helps
him to know how far a theoretical statement is applicable in
practical life.